This invention relates to the production of low dielectric loss ceramic materials, particularly ceramic materials useful in dielectric resonators. In current microwave communication, technology dielectric resonators (DRs) are key elements for filters, low phase noise oscillators and frequency standards. DRs possess resonator quality factors (Q) comparable to cavity resonators, strong linearity at high power levels, weak temperature coefficients, high mechanical stability and small size.
Ceramic dielectric materials are used to form thermally stable DRs as key components in a number of microwave subsystems which are used in a range of consumer and commercial market products. These products range from Satellite TV receiver modules (frequency converter for Low Noise Broadcast (LNB)), Cellular Telephones, PCN""s. (Personal Communication Networks Systems) and VSAT (Very Small Aperture Satellite) systems for commercial application to emerging uses in transportation and automobile projects, such as sensors in traffic management schemes and vehicle anti-collision devices. Dielectric Resonators may be used to determine and stabilise the frequency of a microwave oscillator or as a resonant element in a microwave filter. New systems of satellite TV transmission, based on digital encoding and compression of the video signals, determine the need for improved DR components. The availability of advanced materials will also enable necessary advances in the performance of DRs used for other purposes as referred to above.
In this specification, the term xe2x80x98ceramicxe2x80x99 means any solid inorganic particulate material, the particles of which can be caused to sinter together by the application of heat. The term ceramic has been used also to describe single crystals of inorganic materials such as alumina, titania, etc.
Low dielectric loss materials are highly desirable in the area of communications over a wide frequency range. As an example, resonators using dielectric sintered ceramics may be used in base stations required for mobile communications. The materials used are often complex mixtures of elements.
One of the earliest resonator materials was Barium Titanate (BaTiO3 or BaTi4O9 see, for example, T Negas et al American Ceramic Society Bulletin, vol. 72, pp 80-89 1993). The dielectric loss of a material is referred to as the tan delta and the inverse of this quantity is called the Q (Quality Factor). The Q factor of a resonator is determined by choosing a resonance and then dividing the resonant frequency by the band width 3 dB below the peak.
The losses in ceramic materials may be associated with molecules or defects which can be spatially oriented (Debye loss), due to the inertia of free charges, e.g. electrons in a metal or resonant absorption at certain frequencies. It is considered that extrinsic factors such as impurities and e.g. oxygen vacancy concentration as well as microstructure are of overriding importance. Single crystals or xe2x80x98perfectxe2x80x99 crystals have a lower loss than corresponding poly-crystalline materials. The difference between a xe2x80x98perfectxe2x80x99 single crystal and a polycrystalline ceramic are thought to be due to the huge differences in microstructure and perfection between the two and are clear indicators why it is considered impossible to achieve a dielectric loss approaching that of single crystal counterparts in sintered materials.
A single crystal is made from a melt. The melting temperatures of these crystals is extremely high. For example, the melting point of alumina is 2072C, of magnesia 2852C, of zirconia 2700C, of yttria 2410C and titania 1850C.
The particulate ceramic material can be shaped in a variety of ways, for example, by uniaxial powder pressing, by isostatic pressing, by slip-casting or by polymer processing and extrusion. The resultant shape is then sintered at high temperature and this is associated with a shrinkage and a decrease in the volume of the body. The sintering step can take place in air or in special atmospheres be they oxidising, reducing or inert.
Sintering a ceramic involves taking a fine powder of the material, pressing it into the desired shape and then heating it to temperatures less than their melting point (usually about 75% of the melting point). The powders sinter together in an effort to reduce surface energy and this is accomplished by the reduction in surface area until the porosity is reduced substantially or entirely. The sintering process involves less expensive capital equipment and is less energy intensive.
The major problem with dielectric ceramics is that their dielectric loss is much higher than single crystals. Single crystal materials can exhibit very low loss and this is usually attributed to the absence of grain boundaries and the greater perfection in their structure. The problem with single crystals is that they are time consuming to manufacture and they are extremely expensive. For example, a single crystal of alumina in cylindrical form is around 10,000 times more expensive than an identically shaped sintered alumina.
We have now discovered sintered ceramic materials with a low dielectric loss and a method for making them.
The object of the invention is to provide a doped alumina ceramic which has a high Q value. The object is achieved by forming the ceramic from a homogenous mixture of alumina containing less than 165 ppm impurities, less than 1% weight of a metal oxide of a metal of cation ionic radius of 0.55 to 1, in which the alumina has a particle size of 0.01 to 2 microns and a density of at least 98% theoretical.
The metal or semi-metal oxide, which forms the minor part of the materials of the invention, are preferably oxides of elements of Group III and IV of the periodic table such as Ti, Nb, Y and Zr. The minor part is preferably present in an amount of less than 2% by weight of the total weight of the composition and, more preferably, less than 1% by weight.
The materials of the present invention can be made by homogenously mixing the alumina powder and the dopant powder. In order to achieve homogenous mixing the particle size of the dopant should be the same or less than that of the alumina powder. Alternatively homogenous mixing can achieved by precipitating from solution. The mixed powders can be formed into the ceramic by mixing the powders of alumina and the minor component, for example, of particle size of 0.01 to 2 microns and pressing the mixture into a shape and then heating to a temperature below its melting point, typically 75% of the melting point. The powders sinter together until the porosity is substantially or entirely reduced. Preferably the temperature of sintering for powders such as alumina is less than 1600xc2x0 C. and more preferably between 1500xc2x0 C. and 1600xc2x0 C. In place of the metal oxide used as the dopant a salt of the metal which forms the oxide on the conditions of forming the ceramic can be used e.g. the metal carbonate.
It is very surprising that the addition of dopant oxides with a low Q factor to alumina can increase the Q factor of the resulting material very considerably and can approach the value of single crystals. It was considered that any material with grain boundaries must inevitably show high dielectric loss.
The dielectric loss of polycrystalline sintered alumina has been measured by several workers and the results vary widely. For example, Ceramic Source, vol. 6 1990, American Ceramic Society (publ) reports the loss of alumina as Q=1000 at 10 GHz and 25xc2x0 C.
The highest Q previously measured in alumina at room temperature (i.e. approximately 25xc2x0 C.) is by Woode et al (R A Woode, E N Ivanov, M E Tobar and D G Blair xe2x80x98Measurement of dielectric loss tangent of alumina at microwave frequencies and room temperaturexe2x80x99 Electronics Letters, vol. 30 no. 25, Dec. 8, 1994) who measured a Q of 23,256. This Article noted that purity alone was a poor indicator of the dielectric loss tangent. We have found that although an impure alumina will give a poor Q, a very pure alumina is not a guarantee of a high Q. Pure alumina can have compounds added to it in order to assist the sintering process. These additions should not adversely influence the Q. So, for example, magnesia may be added to a very pure alumina and this will assist the sintering but will not adversely affect the Q if added in small quantities. However, adverse effects are observed with impurities such as alkali salts (sodium and potassium) and metallic elemental impurities such as iron.
The invention also comprises a sintered ceramic material comprising an alumina and minor amount of a metal or semi-metal oxide which has a Q value greater than 25,000, more preferably greater than 30,000 and even more preferably greater than 45,000 at 9 to 10 GHz and at 25xc2x0 C.
The invention is described in the following Examples.
The aluminas used were commercially available aluminas and the analyses of the powders used in the examples are given in Table 1 with the impurities given in parts per million based on the total weight of the sample.
The aluminas used had a particle size of 0.01 to 2 microns.